1. Field of the Invention
The present invention relates to a process for cleaning a harmful gas. More particularly, it pertains to a process for cleaning a harmful gas containing a nitrogen fluoride, especially nitrogen trifluoride which harmful gas is exhausted from a semiconductor manufacturing process.
2. Description of the Related Arts
With continuous development in semiconductor industry, there has been in recent years, a steady rise in the use amounts of nitrogen fluorides, especially nitrogen trifluoride, which are used for the dry etching of silicon, a silicon compound, a tungsten compound and the like and are also used as a cleaning gas for a chamber of CVD equipment.
Nitrogen trifluoride has been reported to be sparingly soluble in water, considerably stable at room temperature hardly reacting with an acid or an alkali, but highly toxic, and thus have a threshold limit value in time weighted average (TLV-TWA) of 10 ppm. Therefore, in the case where nitrogen trifluoride is exhausted into the atmosphere, it exerts evil influence upon human bodies and environment. Accordingly, nitrogen trifluoride needs to be cleared soon after a nitrogen trifluoride-containing gas is used in a semiconductor manufacturing process prior to the exhaust thereof into the atmosphere.
Nitrogen trifluoride, although being stable at ordinary temperature, forms dinitrogen tetrafluoride, dinitrogen difluoride, dinitrogen hexafluoride, fluorine and the like owing to heating, electric discharge or the like during the step of etching or cleaning. In addition, in the case where etching is carried out under the condition in which nitrogen trifluoride is in contact with silicon, tungsten, a silicon compound, a tungsten compound or the like, the nitrogen trifluoride results in the formation of silicon tetrafluoride and tungsten hexafluoride. The afore-said nitrogen fluorides, fluorine, silicon tetrafluoride and tungsten hexafluoride are each more toxic than nitrogen trifluoride, and therefore must be removed as is the case with nitrogen trifluoride.
As processes for removing nitrogen fluorides that are contained in a gas, there have heretofore been proposed various processes including 1 a process in which a nitrogen fluorides-containing gas is brought into contact with metallic silicon at a temperature not lower than 100.degree. C. Japanese Patent Application Laid-Open No.12322/1988 (Sho 63)!; 2 a process in which a nitrogen fluorides-containing gas is brought into contact with metallic titanium at a temperature not lower than 200.degree. C. Japanese Patent Publication No.48571/1988 (Sho 63)!; and 3 a process in which a nitrogen fluorides-containing gas is brought into contact with any of silicon, boron, tungsten, molybdenum, vanadium, selenium, tellurium, germanium and non-oxide base compounds thereof at a temperature in the range of 200 to 800.degree. C. Japanese Patent Publication No.48570/1988 (Sho 63)!.
Also there have been proposed 4 a process in which a nitrogen fluorides-containing gas is brought into contact with a metal halogenide capable of halogen exchange with nitrogen trifluoride Japanese Patent Publication No. 48569/1988 (Sho 63)!; 5 a process in which a nitrogen fluorides-containing gas is brought into contact with an oxide of a transition metal such as iron, manganese and copper at a temperature not lower than 250.degree. C. Japanese Patent Application Laid-Open No.181316/1991 (Hei 3)!; and 6 a process in which a nitrogen fluorides-containing gas is brought into contact with activated carbon at a temperature in the range of 300 to 600.degree. C. Japanese Patent Application Laid-Open No.23792/1987 (Sho 62)!.
Moreover there have been proposed 7 a process in which a nitrogen fluorides-containing gas is brought into contact at a temperature not lower than 200.degree. C. with a catalyst containing a transition metal such as nickel, iron and cobalt or a noble metal such as platinum, rhodium and palladium, said metals being supported on a carrier comprising alumina or silica as a principal component Japanese Patent Application Laid-Open No.273039/1987 (Sho 62)!; 8 a process in which a nitrogen fluorides-containing gas is brought into contact with zirconium or a zirconium-based alloy at a temperature in the range of 100 to 800.degree. C. Japanese Patent Application Laid-Open No.238128/1994 (Hei 6)!; and like processes.
However, the above-mentioned processes suffer the disadvantages or inconveniences that in the aforesaid processes 1, 2 and 3, a volatile fluoride is newly formed as a by-product during cleaning; in the process 4, a halogen gas such as chlorine gas is formed as a by-product; and in the process 5, a nitrogen oxide, which is formed as a by-product, makes it imperative to treat itself, thereby not only complicating the process, but also increasing the treatment cost.
In addition, the process 6 involves the problem that the treatment at an elevated temperature is accompanied by not only violent reaction but also a danger of explosion and besides, carbon tetrafluoride is formed as a by-product which is relatively stable and difficult to remove.
Moreover, the process 7 involves the problem that the treatment, although not being accompanied with the by-production of a harmful gas, must be carried out at a high temperature for the purpose of assuring a sufficient cleaning capacity, for example, the use of iron as a transition metal requires heating to 400.degree. C. or higher in order to obtain a practical cleaning capacity, since the decomposition activity of iron is not sufficiently high at 200.degree. C. The process further involves the problem that some fluorides which are formed with the progress of the reaction cover the surface of a cleaning agent, thereby preventing the reaction from advancing to the inside of the agent resulting in failure to obtain sufficient cleaning capacity.
Further, the process 8 has the hazard of causing uncontrollable rapid temperature rise due to the violent exothermic reaction at a high temperature with nitrogen that is usually used as a diluent gas for nitrogen trifluoride.
As described hereinbefore, there has not yet been found out a satisfactory process for removing nitrogen fluorides that are contained in a gas. In such a circumstance, it has been eagerly desired to develop a process for cleaning nitrogen fluorides which has a high treatment capacity at a low temperature without by-producing a harmful gas or a gas having a fear of environmental pollution.